David Wardale’s “Red Devil” book explains the “grate limit” of a coal-fired steam locomotive. Namely, as you fire more coal on a given size grate, more of it gets entrained in the draft and carried out the stack before it finishes burning. At the maximum output of a locomotive limited by this grate limit, fully half of the coal consumed does not contribute to the fire but rather gets carried out the stack and sprinkled about the right-of-way.
Livio Dante Porta’s Gas Producer Combustion Systems (GPCS), which Wardale implemented on the Red Devil in South Afric and was less successful implementing on the QJ class in China, is intended to improve efficiency by dramatically reducing carryover. The idea is to limit the air flow through the grate, using that primary air to gasify the coal rather than burn it completely. This reduced primary air flow cuts down on entrainment of the coal particles as they burn down to small size. Secondary air is then introduced above the firebed so as to complete the combustion of the gasified coal.
Wardale completes his book by expressing frustration at getting the gas producer combustion process to work reliably, and strongly hints that the ultimate solution would lie with pulverized coal rather than trying to burn lump coal on a grate. With pulverized coal, the coal is ground so fine that it burns on contact with the hot firebox air much as the atomized oil in an oil burning firebox or furnace.
Pulverized coal on a locomotive has its host of problems, an important one is that it is pulverized before being put into the tender, creating the problem of how to safely store it without and explosion hazard. Pulverized coal has had mixed success in the few instances it has been tried.
But oil burning steam locomotives have been used wherever there was a cheap enough supply of the heavy “bunker” oil traditionally used for this purpose.
David Wardale’s “Red Devil” book explains the “grate limit” of a coal-fired steam locomotive. Namely, as you fire more coal on a given size grate, more of it gets entrained in the draft and carried out the stack before it finishes burning. At the maximum output of a locomotive limited by this grate limit, fully half of the coal consumed does not contribute to the fire but rather gets carried out the stack and sprinkled about the right-of-way.
Livio Dante Porta’s Gas Producer Combustion Systems (GPCS), which Wardale implemented on the Red Devil in South Afric and was less successful implementing on the QJ class in China, is intended to improve efficiency by dramatically reducing carryover. The idea is to limit the air flow through the grate, using that primary air to gasify the coal rather than burn it completely. This reduced primary air flow cuts down on entrainment of the coal particles as they burn down to small size. Secondary air is then introduced above the firebed so as to complete the combustion of the gasified coal.
Wardale completes his book by expressing frustration at getting the gas producer combustion process to work reliably, and strongly hints that the ultimate solution would lie with pulverized coal rather than trying to burn lump coal on a grate. With pulverized coal, the coal is ground so fine that it burns on contact with the hot firebox air much as the atomized oil in an oil burning firebox or furnace.
Pulverized coal on a locomotive has its host of problems, an important one is that it is pulverized before being put into the tender, creating the problem of how to safely store it without and explosion hazard. Pulverized coal has had mixed success in the few instances it has been tried.
But oil burning steam locomotives have been used wherever there was a cheap enough supply of the heavy “bunker” oil traditionally used for this purpose.
LaMassena’s article on the “Big Engines” in the June 1968 issue of Trains hinted that for a given grate area, that oil firing would generate less steam than firing with a high quality bituminous coal.
The coal particles flying around the firebox should have been excellent sources of radiant energy, presumably more so than with the cleaner flame from oil firing. I would not be surprised if radiant heat transfer was the limiting factor in steam production as opposed to how much energy was released by combustion. (It is quite possible that I may be way off base on this supposition.)
Oil as a fuel certainly has advantages over coal, such as more BTU’s, easier handling, no ashes to dump, no problems with spontanious combustion as coal can have from the dust.
But there’s a good story from “Classic Trains” that’ll show you everything wasn’t always “hunky-dory” on oil-fired steamers.
Go to the “Classic Trains” site, select “The Way It Was” and scroll down to “The Challenges Of Firing An Oil Burner” by Barry Anderson.
Coal fire carryover appeared in the form of raining cinders and hot sparks. Oil fire carryover (the result of a too-rich fuel-air mixture) appeared in a towering black cloud that blew away on the wind.
There are plenty of photos of steam locomotives, both coal and oil fired, fouling the air with dirty exhaust. Nor are diesels exempt. A lot depended on the quality of the fuel. Even more depended on the skill of the fireman.
When I was a teen, I rode on a CB&Q steam excursion in 1962 or 63 from CUS to Starved Rock State Park (on the line to the Twin Cities). It was pulled by 5632, an oil-burning 4-8-4. Much of the great trip, I rode in the open on a trailing gondola. When I returned home, I was largely covered in oily soot. http://www.rrpicturearchives.net/pictures/16736/CBQ5632-630428%20Hinsdale%20(Highlands),%20IL%201.jpg
The smoke from a coal-fired steam engine is not the same thing as the cinders (also called sparks), which I believe is the carryover of carbon from the stack on to the right-of-way. According to Wardale, a locomotive can smoke badly, but this is mainly visual and does not represent much wasted fuel. The carbon carryover is largely invisible, except at night when the bigger “sparks” are visible as glowing embers.
The shower of cinders from sitting in the gondola behind a coal-fired steamer is the carbon carryover. I suppose there could be that much oily soot behind an oil burner, but in excursion service, the try to smoke things up for an impressive photo run by.
No one seems to have any fuel consumption figures of any modern oil burner – say those huge AT&SF 4-8-4’s?
If I’m not mistaken, there is good data in the Farrington ‘Santa Fe Big Three’ regarding oil firing rate and conditions. Specifically I remember a table with the pressure in the firebox space and chamber for, I believe, the 3460 class Hudsons. Note that most of the pressures are below atmospheric, and yet characterized by high rates of combustion. The references I have on oil firing indicate that a minimization of excess air (to not more than a few percent) over the range of firing is desirable. All this sums up to unburned carbon.
An advantage of oil firing is that the ‘atomization’ of the fuel is more easily accomplished, and entrainment and carburetion of the fuel in the primary and secondary air is easier… up to a point. When the hydrogen has flashed off the fuel droplets, you are back to carbon – hot carbon, yes, but not acting much like a liquid. After that point expect it to act just like similarly-sized levitated coal particles.
There are two things going on at a given point in the combustion gas plume. There is radiation from the heat of each particle, and there is combustion from reaction of oxygen at the surface of each particle. The amount of available oxygen is limited, and the amount of ‘scrubbing’ that moves carbon dioxide away from
I was not clear. Sorry. #5632 class O-5B was a modern (1940) oil-burning 4-8-4. TE = 67541 lbs. Most of the run, it ran quite clean. Only in the obligatory run-bys for the debarked passengers at Starved Rock did it put on a ‘show’ of heavy black smoke
It doesn’t take long for an oil-fired smoke show to coat you in fallout! (I suspect that more than a little sanding of the flues would produce similar effect, perhaps with some grit mixed in…)
As I understand it, typical firing practice involved just a little ‘overfiring’, to produce a light gray haze at the stack. (This is what the stack light would show at night.) I do not know how this might tend to ‘plate out’ downwind…
Yes, I understand. Sanding the flues was, as I was told, the tactic used to produce a show of black smoke. However, I was standing rather far from the rails during the run-by. The accumulation I had was mostly from the trip out, as I rode inside a coach on the return to Aurora.